INSECTIVOROUS PLANTS 187 
epidermis or skin of the leaf. The tentacles of 
Drosera have a complicated structure resembling that 
of leaves, and the tip is occupied by a gland which 
produces the sticky secretion already mentioned. 
These glands are exceedingly sensitive, and, more- 
over, sensitive in a selective way. They are unaffected 
by the drops of rain which frequently fall on them, 
but the touch of any solid body, especially of organic 
material, immediately affects them; most of all nitro- 
genous substances of any kind. Darwin found that 
a morsel of human hair weighing only ;s:+20 of a 
grain was sufficient to set the machinery of Drosera 
in motion, and that immersion of a leaf in a solution 
of phosphate of ammonium so weak that each tentacle 
could absorb only zso0v000 of a grain acted as a 
strong stimulus. In nature the stimulus is usually 
given by some unwary insect—a midge or other small 
flying creature—which, attracted by the bright colour 
or by the odour of the leaf, ventures too close, and 
becomes entangled among the sticky hairs. Then a 
most interesting series of events takes place. Almost 
at once the tentacles—first the ones actually touched, 
and then the adjoining ones—bend towards the point 
of disturbance, closing down one by one on the 
unfortunate victim till the leaf resembles a closed fist. 
At the same time the production of secretion increases, 
so as further to entangle the victim. When it is 
firmly secured, the secretion changes in character. 
Digestive ferments, closely resembling those by which 
animals digest their food, are poured out. These 
dissolve the animal’s body, all except the horny parts; 
the digested materials are then absorbed into the 
plant, which, as experiments show, benefits consider- 
